Nickel silicide - silicon nitride adhesion through surface passivation

ABSTRACT

A process for forming nickel silicide and silicon nitride structure in a semiconductor integrated circuit device is described. Good adhesion between the nickel silicide and the silicon nitride is accomplished by passivating the nickel silicide surface with nitrogen. The passivation may be performed by treating the nickel silicide surface with plasma activated nitrogen species. An alternative passivation method is to cover the nickel silicide with a film of metal nitride and heat the substrate to about 500° C. Another alternative method is to sputter deposit silicon nitride on top of nickel silicide.

FIELD OF THE INVENTION

This invention relates to the field of semiconductor integrated devices,and particularly relates to a process of surface passivation forimproving adhesion between a nickel silicide layer and a silicon nitridelayer.

DESCRIPTION OF THE RELATED ART

The performance of a silicon integrated circuit is related to itscomponent size. Small components enable more integrated functionalityand faster operation. One technical challenge to the engineers in theirendeavor to reduce the silicon integrated circuit component size is tomaintain the conductance of the conductive lines that hook up thecircuit components. One type of conductive line currently used in thesilicon integrated circuit industry is refractory metal silicide cladsilicon. For example, cobalt silicide clad silicon is the material ofchoice in making MOS transistor gate electrodes and the source and drainregion of high-performance logic circuits.

Cobalt silicide clad silicon served its designed purpose well. Butcobalt silicide process encounters problems when polysilicon linebecomes narrower than 50 nanometers. One problem is that cobalt siliciderequires a temperature of about 700° C. to form, which over taxes thethermal budget of total process. Another problem is at 700° C., grainsin the polysilicon conglomerate, which reduces the polysilicon volumeand causes the formation of voids in the polysilicon lines. Depending onthe width of the polysilicon line and the density and the size of thevoids, the conductance of the polysilicon line may vary substantially.

To overcome the shortcomings of cobalt silicide, engineers turn tonickel silicide. Nickel reacts with silicon and forms nickel silicide ata temperature below 300° C. We have demonstrated that between 260° C.and 310° C., we can form nickel silicide alloy that is rich in nickeland has stable sheet resistance with good uniformity. The resistivitydrops further when the alloy is treat at between 400° C. to 550° C. andthe alloy converts substantially to nickel mono-silicide. The loweringof process temperature to about 500° C. effectively eliminates orsubstantially reduces the problem of polysilicon grain conglomerationand substantially conserves the thermal budget of the total process.

There are problems, however, that keep the nickel silicide cladpolysilicon process from widely implemented. One known problem is thatthe insulation layer, usually a silicon nitride film, which forms on thesurface of the nickel silicide, has a tendency to blister or peel offfrom the silicide surface. This poor adhesion of this insulation layercauses undesirable excessive power consumption and reliability problem.

Engineers in semiconductor equipment manufacturing companies andintegrated circuit manufacturing companies have been trying to solve theadhesion problem without success. The present invention effectivelyeliminates or substantially reduces this problem.

SUMMARY OF THE INVENTION

We have determined that the root cause of the poor adhesion between thenickel silicide and the silicon nitride is the presence of a siliconrich interface film. The present invention eliminates or substantiallyreduces the adhesion problem by preventing such film from formation.

In the known art, a thin silicon nitride layer is usually provided incombination with a thicker silicon dioxide film for insulating theconductive nickel silicide material electrically from other conductivematerials. It is usually formed in a plasma reactor with a plasmaenhanced chemical vapor deposition (PECVD) process.

The environment of the reactor is a gaseous mixture comprises ammoniaand silane. Energy in the form of radio frequency signal activatesmolecules of silane and ammonia in the plasma and produces silicon andnitrogen species. Silicon and nitrogen species react and form siliconnitride on the surface of the semiconductor substrate.

In the presence of nickel silicide, however, silane decomposes andproduces silicon without the aid the radio frequency signal. Without anabundance of activated nitrogen to form the desired silicon nitride, thesilicon species precipitates on the substrate surface and forms asilicon rich film that is the cause of the adhesion problem.

The present invention solves this problem by preventing the formation ofthis silicon rich film. In one embodiment of the present invention,ammonia is first activated by the radio frequency signal in the absenceof silicon carrying gas. The nitrogen species from the activated ammoniapassivates the nickel silicide surface. Subsequently, silane or othersilicon carrying gas may be mixed in the reactor to form siliconnitride.

In another embodiment of the present invention, nitrogen gas is firstactivated by the radio frequency signal in the absence of siliconcarrying gas. The nitrogen species from the activated nitrogen gaspassivates the nickel silicide surface. Subsequently, silane or othersilicon carrying gas may be mixed in the reactor to form siliconnitride.

In yet another embodiment of the present invention, the semiconductorsubstrate is deposited with a film of titanium nitride or othertransition metal nitride. The nitrogen species in the nitride filmreacts with the nickel silicide at an elevated temperature around 500°C. to passivate the nickel silicide surface. Once the surface isadequately passivated and the residual metal nitride layer is removed,silicon nitride film formation may proceed.

In yet another embodiment of the present invention, a physicalsputtering process deposits the silicon nitride film on thesemiconductor substrate where the target comprises silicon nitridematerial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the cross section of a MOS transistor where a siliconrich film is present between the nickel silicide and silicon nitride.

FIG. 2 is a transmission electron micrograph of a structure comprisesnickel silicide, silicon rich film and silicon nitride.

FIG. 3 is a XPS representation of the percentage composition of thestructure in FIG. 2.

FIG. 4 depicts the cross section of a MOS transistor where the siliconrich film is absent between the nickel silicide and silicon nitride.

FIG. 5 is a transmission electron micrograph representation of a nickelsilicide and silicon nitride absent of a silicon rich film.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENT

FIG. 1 depicts a silicon CMOS transistor 10 manufactured with a knownprocess of nickel silicidation in which a silicon substrate 20 isprovided. The active areas where transistors of the integrated circuitare designated are covered with a thin layer of silicon dioxide 30. Apolysilicon film 40, in the form of aggregated polycrystalline grains ofa desired grain size, is deposited on the silicon substrate. Thepolysilicon film may be doped with proper chemical elements such asphosphorous, boron, and arsenic to achieve a desired electricalresistivity. Silicidation reduces the resistivity still further.

The polysilicon is patterned and etched to form a line pattern accordingto the integrated circuit design. With the photoresist pattern as ashield to cover the line pattern, the portion of polysilicon that isuncovered is removed, usually with a plasma enhanced chemical etchprocess.

Following the polysilicon line formation, pockets of the siliconsubstrate are doped by ion implant to form the source-drain regions 60.The formation of the source-drain regions usually includes the formationof insulation materials 50 and 55 on the sidewalls of the polysiliconlines. In this example, material 50 is silicon nitride and material 55is silicon dioxide. The insulation materials are referred to as thesidewall spacers. The sidewall spacers serve a multitude of functions,one of which is to separate the source-drain regions 60 electricallyfrom the polysilicon line 40 that forms the gate of the transistor 100.

With the polysilicon lines 40 and the sidewall spacers 50 and 55 inplace, a layer of nickel film is applied to the top of the substrate.The nickel film makes contact to the top surface of the polysiliconlines, the sidewall spacers and the surfaces of the source and drainregions, which comprise silicon material.

The silicon substrate with the deposited nickel film then goes through atwo-step heat-treatment process. The temperature at the first step isabout 260° C. to 310° C. In this temperature range, nickel reacts withpolysilicon lines 80 and the source-drain regions 70 and forms anickel-rich silicide alloy. Nickel does not react, however, with thesidewall spacers, which are made of dielectric material. Residual nickelis removed from the substrate surface after the first stepheat-treatment. The temperature of the second step is about 400 to 550°C. In this temperature range, the nickel rich silicide alloy converts tonickel mono-silicide with nickel and silicon atomic number ratio closeto one.

Nickel silicide clad polysilicon makes one type of a plurality ofconductive line means in a modern integrated circuit. Other typesinclude aluminum, tungsten, and copper and other metal alloys and theyare disposed in different geometric planes of the integrated circuit.Nickel silicide clad polysilicon lines are usually connected to theother types of conductive lines by conductive plugs placed in via-holesthrough insulation layers that otherwise separate the silicide fromother conductive lines.

The insulation layers typically include a silicon nitride layer 90 inthe order of 20 to 40 nanometers that lies immediate on the silicide anda silicon oxide layer 100 that may be over 1 micrometer thick and sitson top of the nitride layer. One purpose for employing a two-layeredinsulation is for process control, specifically, the nitride layerserves as etch stop during via-hole formation.

The via-holes are formed with pattern and etch process known to thosewith ordinary skill in the art of integrated circuit manufacturing. Thesilicon oxide layer is partially uncovered by a pattern of photoresist.Etching is by reacting fluorine species activated in a plasma with theuncovered silicon oxide. Because fluorine reacts much slower withsilicon nitride than with silicon oxide, the etching process effectivelystops once it advances through the silicon dioxide layer and uncoversthe silicon nitride film.

The silicon nitride film 90 is formed with a plasma enhanced chemicalvapor deposition (PECVD) process in the known art in which siliconspecies reacts with nitrogen species in a plasma environment and forms asilicon nitride on the silicon substrate. The silicon species may comefrom gaseous silane and the nitrogen species from ammonia. When thesilicide alloy is formed with refractory metals such as titanium orcobalt, the silicon nitride film such formed adheres satisfactorily tothe silicide surface. When the silicide alloy is formed with nickel,however, an unexpected phenomena takes place at the silicide surface:gaseous silane dissociates prematurely into silicon under the processcondition and the silicon species forms a silicon-rich film 85 on thesilicide surface.

As depicted in FIG. 2, the existence of a silicon rich film 285 on thenickel silicide 280 has adverse effect on the silicon nitride 290adhering to the silicide 280 and this degrades the integrity of thestructure because the silicon nitride film 290 may blister and peel offfrom the structure.

FIG. 3 depicts the XPS depth file of the film 285 in FIG. 2 at thevicinity of area 201. From the figure, one can ascertain that the film285 is rich in silicon 310 and is virtually void of nitrogen 320.

FIG. 4 depicts a silicon CMOS transistor 110 manufactured with processesof this invention. Note the absence of a film between regions 170, 180and the silicon nitride film 190.

FIG. 5 is a transmission electron micrograph of a structure thatcomprises silicon 540, nickel silicide 580, and silicon nitride 590.FIG. 5 depicts the result of a process of the present invention. Notethe absence of a silicon-rich film between nickel silicide 580 and thesilicon nitride 590.

In the following sections, embodiments of this invention by way ofexamples are described.

nIn the first embodiment, a nickel silicide is provided on the siliconsubstrate with the two-step heat-treatment process as described above.In order to prevent the silicon rich layer 285 as shown in FIG. 2 fromforming, a process step is added to the silicon nitride depositionprocess. In this step, ammonia or other nitrogen carrying gaseousmaterial such as nitrogen gas is introduced into the plasma reactorchamber. The gaseous atmosphere in the reactor chamber is substantiallyfree of silicon species. Plasma is then induced by the application of aradio frequency signal that in term activates the nitrogen carryinggaseous material such as ammonia. The active nitrogen species reactswith the nickel silicide surface and passivates the nickel silicidesurface.

With the nickel silicide surface thus passivated, silicon nitridedeposition may continue as described above. The silicon nitridedeposition may be carried out in the same plasma reactor chamber or in aseparate reactor chamber.

In the second embodiment, the first step of the two-step silicidationheat-treatment process is performed as described, that is, heat-treatthe substrate at 260 to 310° C. to form a nickel rich alloy at source,drain and polysilicon line regions and the residual nickel is removed.Before the second step of silicidation, a metal nitride film such astitanium nitride is disposed on the substrate, covering the nickelsilicide. The second step of the heat treatment is then performed at 400to 550° C. to convert the nickel rich alloy to a nickel mono-silicidealloy. During the second step of heat treatment, the nitrogen from themetal nitride film gets incorporated in the nickel silicide surface andthus passivates the nickel silicide. The titanium nitride film issubsequently removed from the surface of the substrate before thesilicon nitride film deposition.

This invention is applicable to integrated circuits manufactured onsubstrates other than silicon, for example, on silicon on insulator(SOI), or silicon germanium (Si-Ge) substrate. It also applies tointegrated circuit whose components comprise other than MOS transistorssuch as, for example, bipolar transistors, diodes, capacitors, andresistors. It also applies to electric devices other than integratedcircuits such as, for example, heaters.

1-14. (canceled)
 15. A semiconductor device, comprising a. a substratehaving a top surface that contains a silicon region; b. a silicideregion having a top surface, formed outwardly from the top surface ofthe silicon region; c. a nitride region having a bottom surface, formedoutwardly from the silicide region; and d. an interface between thenitride bottom surface and the silicide top surface that issubstantially free of a silicon rich material that is substantiallydevoid of nitrogen.
 16. A semiconductor device of claim 15, where in thesilicide region contains nickel.
 17. A semiconductor device of claim 15,wherein the nitride region contains silicon.
 18. (canceled)